calibrating speedometer

i have a 904 trans. my car had a 7 1/4 with 2.76 gears. i recently upgraded it to a 8 1/4 with 3.21 gears.

my problem is that my speedometer is off now. does anybody know how i calibrate the speedometer so that it is at least close?

my tires are 26.6 inches tall.

thanks